1. Field of the Invention
This invention relates to a lead-free multi-layer sliding part and to a method for its manufacture. Examples of a sliding part according to the present invention are a cylindrical sliding part such as a bushing for use in a radial sliding bearing (also called a journal bearing) and a planar sliding part for use as a swash plate in a compressor, pump, or hydraulic motor.
2. Description of the Related Art
Sliding bearings are often made of a multi-layer sliding material comprising a backing plate of steel and a layer of a bearing metal bonded to the backing plate in order to increase the strength of the bearings so that they can withstand high loads.
Methods for joining a bearing metal layer to a steel backing plate include the cladding method, the casting method, the flame coating method, and the powder metallurgy method.
In the cladding method, the bonding strength between the bearing metal and the steel backing plate is relatively low, so when a high load is applied to a sliding bearing made from such a multi-layer material, there are cases in which the bearing metal layer peels off the steel backing plate.
In the casting method, a molten bearing metal is cast atop a steel backing plate. Although a bearing metal layer formed by the casting method has a strong bonding strength to the steel backing plate, this method is very troublesome to perform, and at the time of casting of the molten bearing metal, oxides can be included or shrinkage cavities can form, and these may have an adverse effect on bearing properties.
In the flame coating method, a molten bearing metal is blown against a steel backing plate by a high pressure gas, and due to its kinetic energy, the bearing metal burrows into the steel backing plate to form a bearing metal layer. The bearing metal forms a mechanical bond with the backing plate rather than a metallurgical bond, so the bonding strength between the bearing metal layer and the backing plate may be inadequate, and there are cases in which the bearing metal layer peels off the backing plate during use.
In the powder metallurgy method, a bearing metal in the form of a powder is dispersed on a steel backing plate and is sintered to join the grains of powder to each other and to the backing plate. A multi-layer material which is obtained by the powder metallurgy method has a strong bonding strength between the sintered bearing metal layer and the steel backing plate, and the sintering temperature can be made low, so there are few thermal effects on the steel backing plate or the bearing metal such as can occur with the casting method.
The materials which have been most commonly used to form a bearing metal layer of a multi-layer sliding bearing have been Cu-based alloys such as lead bronze, phosphor bronze, and high strength brass.
Lead bronze has good bearing properties, but it is a potential source of environmental pollution. When equipment containing a multi-layer bearing is discarded, the bearing is usually disposed of by burial underground, since the bearing metal layer and the steel backing plate cannot be readily separated from each other and so cannot be easily recycled. If a multi-layer bearing containing lead bronze which has been disposed of in a landfill is contacted by acid rain, lead may be dissolved from the bearing metal layer by the acid rain and may pollute underground water. If underground water which has been polluted by lead in this manner enters the water supply and is drunk for long periods by humans or livestock, the lead accumulates in the body and may cause lead poisoning. For this reason, the disposal of lead-containing materials is being increasingly regulated, and there is a trend in industry away from the use of lead-containing materials such as lead bronze for bearing metal layers of sliding bearings.
Phosphor bronze and high strength brass do not contain lead, but their sliding or bearing properties may not be adequate when they are subjected to high instantaneous loads, such as are applied to a swash plate of a compressor or hydraulic motor at the start of operation. As a result, these materials are more susceptible to seizing.
When a sliding part made of a multi-layer sliding material having a bearing metal layer bonded to a steel backing plate is manufactured by the powder metallurgy method, it is thought to be possible to improve the sliding properties of the bearing metal layer by incorporating a solid lubricant into the bearing metal layer. For this purpose, a powder of a bearing metal is mixed with a powder of a solid lubricant, such as graphite or molybdenum disulfide (MoS2), and the mixed powder is used to form the bearing metal layer by sintering.
However, such a sliding part having a sintered bearing metal layer containing a solid lubricant has the problem that the bearing metal layer may be cracked by an impact when subjected to a high load, thereby causing the bearing metal layer to peel off partly or completely or to wear out prematurely, leading to the occurrence of seizing.
JP-A 55-134102 (1980) describes forming a bearing by sintering of a mixture of metal powders and a Cu-plated solid lubricant powder. According to the technique disclosed therein, metal powders (such as Cu powder and Sn powder) which form a bearing metal are mixed with a Cu-plated solid lubricant powder (such as Cu-plated graphite powder or Cu-plated molybdenum disulfide powder). The powder mixture is subjected to cold or hot pressing in a mold and then sintered to form a Cu-based sintered bearing. The sintered body may be machined so as to form a bore and obtain a cylindrical bearing. However, this sintered bearing exhibits large variations in bearing properties, and if it is used as a bearing metal layer of a multi-layer bearing, its mechanical strength is too low for it to be used as a bushing for construction equipment on which a very high load is exerted. The use of Cu-plated graphite to form a sintered bearing material is also described in JP-A 05-248441 (1993).
The present inventors investigated the problems of conventional multi-layer sliding parts, such as sliding bearings, in order to provide a lead-free multi-layer sliding part in which these problems are eliminated or alleviated, and found the following.
The reason for the seizing which is encountered with a bearing made of phosphor bronze and high strength brass is that these bearing metals do not have good lubricity in the absence of a solid lubricant.
A bearing metal layer containing a solid lubricant formed by sintering a powder mixture of a bearing metal and a solid lubricant on a steel backing plate has improved lubricity, but it undergoes cracking and peeling because the particles of solid lubricant do not metallically bond to either the bearing metal or to the steel backing plate, so the bearing metal layer formed by sintering has poor bonding strength.
This problem can be alleviated to some extent by employing the technique disclosed in JP-A 55-134102 (1980), i.e., by plating the solid lubricant powder with copper and using the resulting Cu-plated solid lubricant powder to form a lubricant-containing bearing metal layer by sintering, since the Cu-plated solid lubricant powder can metallically bond to the bearing metal. However, according to the technique disclosed therein, various powders including a Cu powder, a Sn powder, and a Cu-plated solid lubricant powder are used to form a powder mixture to be sintered, and due to the differences in specific gravity of these powders, it is difficult to form a uniform mixture by mechanical mixing, and this results in a fluctuation in the bearing properties of the resulting sintered bearing layer. In addition, the mechanical strength of the sintered bearing metal layer is not sufficient to withstand a very high load. There is no bonding between the steel backing plate and the sintered bearing metal layer.